Sodium/molybdenum composite metal powders, products thereof, and methods for producing photovoltaic cells

ABSTRACT

A method for producing a metal article may include: Producing a supply of a composite metal powder by: providing a supply of molybdenum metal powder; providing a supply of a sodium compound; combining the molybdenum metal powder and the sodium compound with a liquid to form a slurry; feeding the slurry into a stream of hot gas; and recovering the composite metal powder; and consolidating the composite metal powder to form the metal article, the metal article comprising a sodium/molybdenum metal matrix. Also disclosed is a metal article produced accordance with this method.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of co-pending U.S. patent application Ser. No.12/013,263, filed Jan. 11, 2008, which is hereby incorporated herein byreference for all that it discloses.

TECHNICAL FIELD

This invention relates molybdenum-containing materials and coatings ingeneral and more specifically to molybdenum coatings suitable for use inthe manufacture of photovoltaic cells.

BACKGROUND

Molybdenum coatings are well-known in the art and may be applied by avariety of processes in a wide variety of applications. One applicationfor molybdenum coatings is in the production of photovoltaic cells. Morespecifically, one type of high-efficiency polycrystalline thin filmphotovoltaic cell involves an absorber layer comprising CuInGaSe₂. Suchphotovoltaic cells are commonly referred to as “CIGS” photovoltaic cellsafter the elements comprising the absorber layer. In a commonconstruction, the CuInGaSe₂ absorber layer is formed or “grown” on asoda-lime glass substrate having a molybdenum film deposited thereon.Interestingly, it has been discovered that small quantities of sodiumfrom the soda-lime glass substrate diffusing through the molybdenum filmserve to increase the efficiency of the cell. See, for example, K.Ramanathan et al., Photovolt. Res. Appl. 11 (2003), 225; John H.Scofield et al., Proc. of the 24^(th) IEEE Photovoltaic SpecialistsConference, IEEE, New York, 1995, 164-167. While such efficiency gainsare automatically realized in structures wherein the CIGS cell isdeposited on soda-lime glass substrates, it has proven considerably moredifficult to realize efficiency gains where other types of substratesare used.

For example, there is considerable interest in forming CIGS cells onflexible substrates so that the cells may be made lighter and may bereadily conformed to a variety of shapes. While such cells have beenmade and are being used, the flexible substrates involved do not containsodium. Consequently, the performance of CIGS cells manufactured on suchsubstrates may be improved by doping the molybdenum layer with sodium.See, for example, Jae Ho Yun et al., Thin Solid Films, 515, 2007,5876-5879.

SUMMARY OF THE INVENTION

A method for producing a metal article according to one embodiment ofthe invention may include: Producing a supply of a composite metalpowder by: providing a supply of molybdenum metal powder; providing asupply of a sodium compound; combining the molybdenum metal powder andthe sodium compound with a liquid to form a slurry; feeding the slurryinto a stream of hot gas; and recovering the composite metal powder; andconsolidating the composite metal powder to form the metal article, themetal article comprising a sodium/molybdenum metal matrix. Alsodisclosed is a metal article produced accordance with this method.

Also disclosed is a method for producing a composite metal powder thatinvolves: Providing a supply of molybdenum metal powder; providing asupply of a sodium compound; combining the molybdenum metal powder andthe sodium compound with a liquid to form a slurry; feeding the slurryinto a stream of hot gas; and recovering the composite metal powder.Also disclosed is a composite metal powder produced according to thisprocess.

Another embodiment for producing a composite metal powder may include:Providing a supply of molybdenum metal powder; providing a supply of asodium molybdate powder; combining the molybdenum metal powder and thesodium molybdate powder with water to form a slurry; feeding the slurryinto a stream of hot gas; and recovering the composite metal powder.Also disclosed is a composite metal powder produced in accordance withthis process.

A method for producing a photovoltaic cell in accordance with theteachings provided herein may include: Providing a substrate; depositinga sodium/molybdenum metal layer on the substrate; depositing an absorberlayer on the sodium/molybdenum metal layer; and depositing a junctionpartner layer on the absorber layer.

A method for depositing a sodium/molybdenum film on a substrate mayinvolve: Providing a supply of a composite metal powder comprisingmolybdenum and sodium; and depositing the composite metal powder on thesubstrate by thermal spraying. Another method for depositing a film on asubstrate may comprise: Sputtering a target comprising asodium/molybdenum metal matrix, sputtered material from the targetforming the sodium/molybdenum film. Another method for coating asubstrate may include: Providing a supply of composite metal powdercomprising molybdenum and sodium; and evaporating the composited metalpowder to form a sodium/molybdenum film. A method for coating asubstrate may include: Providing a supply of a composite metal powdercomprising molybdenum and sodium; mixing the supply of composite metalpowder with a vehicle, and depositing the mixture of the composite metalpowder and the vehicle on the substrate by printing.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative and presently preferred exemplary embodiments of theinvention are shown in the drawings in which:

FIG. 1 is a schematic representation of one embodiment of basic processsteps which may be utilized to produce a sodium/molybdenum compositemetal powder;

FIG. 2 is a process flow chart depicting methods for processing thecomposite metal powder mixture;

FIG. 3 is a enlarged cross-section in elevation of a photovoltaic cellhaving a sodium/molybdenum metal layer;

FIG. 4 is a scanning electron microscope image of a sodium/molybdenumcomposite metal powder mixture;

FIG. 5 a is a spectral map produced by energy dispersive x-rayspectroscopy showing the dispersion of sodium in the image of FIG. 4;

FIG. 5 b is a spectral map produced by energy dispersive x-rayspectroscopy showing the dispersion of molybdenum in the image of FIG.4;

FIG. 6 is a schematic representation of one embodiment of pulsecombustion spray dry apparatus; and

FIG. 7 is a plot showing the screen fraction distributions of exemplarycomposite metal powders produced in accordance with the teachingsprovided herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A process or method 10 for producing a sodium/molybdenum composite metalpowder 12 is illustrated in FIG. 1 and, briefly described, may comprisea supply of a molybdenum metal powder 14 and a supply of a sodiumcompound 16, such as, for example, sodium molybdate (Na₂MoO₄) powder.The molybdenum metal powder 14 and sodium molybdate powder 16 arecombined with a liquid 18, such as water, to form a slurry 20. Theslurry 20 may then be spray dried, e.g., by a pulse combustion spraydryer 22, in order to produce the sodium/molybdenum composite metalpowder 12.

Referring now primarily to FIG. 2, the sodium/molybdenum composite metalpowder 12 may be used in its as-recovered or “green” form as a feedstock24 for a variety of processes and applications, many of which are shownand described herein, and others of which will become apparent topersons having ordinary skill in the art after having become familiarwith the teachings provided herein. Alternatively, the “green” compositemetal powder 12 may be further processed, e.g., by sintering 26, byclassification 28, or combinations thereof, before being used asfeedstock 24. The sodium/molybdenum composite metal powder feedstock 24(e.g., in either the “green” form or in the processed form) may be usedin a thermal spray deposition process 30 in order to deposit asodium/molybdenum film 32 on a substrate 34, as best seen in FIG. 3.Such sodium/molybdenum films 32 may be used to advantage in a widevariety of applications. For example, and as will be described infurther detail below, the sodium/molybdenum film 32 may comprise aportion of a photovoltaic cell 36 and may be used to improve theefficiency of the photovoltaic cell 36. In an alternate depositionprocess, the composite metal powder 12 may also be used as a feedstock24 in a printing process 38 which may also be used to form asodium/molybdenum film or coating 32′ on substrate 34.

In still yet another embodiment, the composite metal powder feedstock24, again in either its “green” form or in its processed form, may beconsolidated at step 40 in order to produce a metal product 42, such asa sputter target 44. The metal product 42 may be used “as is” directlyfrom consolidation 40. Alternatively, the consolidated product may befurther processed, e.g., by sintering 46, in which case the metalproduct 42 will comprise a sintered metal product. In the case where themetal product 42 comprises a sputter target 44 (i.e., in either asintered form or an un-sintered form), the sputter target 44 may be usedin a sputter deposition apparatus (not shown) in order to deposit asodium/molybdenum film 32″ on substrate 34. See FIG. 3.

Referring now primarily to FIGS. 4, 5 a, and 5 b, the sodium/molybdenumcomposite metal powder 12 comprises a plurality of generallyspherically-shaped particles that are themselves agglomerations ofsmaller particles. Accordingly, the composite metal powder 12 may becharacterized herein in the alternative as “soccer balls” formed of“BB's.” Moreover, and as is evidenced by FIGS. 5 a and 5 b, the sodiumis highly dispersed within the molybdenum. That is, thesodium/molybdenum composite powders of the present invention are notmere combinations of sodium metal powders and molybdenum metal powders,but rather comprise substantially homogeneous dispersions or compositemixtures of sodium and molybdenum sub-particles that are fused oragglomerated together. The sodium/molybdenum metal powder composite isalso of high density and possesses favorable flow characteristics. Aswill be discussed in further detail herein, exemplary sodium/molybdenumcomposite metal powders 12 produced in accordance with the teachingsprovided herein may have Scott densities in a range of about 2 g/cc toabout 3 g/cc. Hall flowabilities range from less than about 35 s/50 g toas low as 30 s/50 g for the various example compositions shown anddescribed herein.

A significant advantage of the present invention is that it provides ametallic combination of molybdenum and sodium that is otherwisedifficult or impossible to achieve by conventional methods. Moreover,even though the sodium/molybdenum composite metal powder comprises apowdered material, it is not a mere mixture of sodium and molybdenumparticles. Instead, the sodium and molybdenum sub-particles are actuallyfused together, so that individual particles of the powdered metalproduct comprise both sodium and molybdenum. Accordingly, powderedfeedstocks 24 comprising the sodium/molybdenum composite powdersaccording to the present invention will not separate (e.g., due tospecific gravity differences) into sodium particles and molybdenumparticles. Furthermore, coatings or films produced from thesodium/molybdenum composite metal powders will have compositions thatare similar to the compositions of the sodium/molybdenum metal powderssince such deposition processes do not rely on the co-deposition ofseparate molybdenum and sodium particles that would each have differentdeposition rates.

Besides the advantages associated with the ability to provide acomposite metal powder wherein sodium is highly and evenly dispersedthroughout molybdenum, the composite metal powders disclosed herein arealso characterized by high densities and flowabilities, thereby allowingthe composite metal powders to be used to advantage in a wide variety ofpower metallurgy processes that are now known in the art or that may bedeveloped in the future. For example, the sodium molybdenum compositemetal powders may be readily used in a wide variety of thermal spraydeposition apparatus and associated processes to depositsodium/molybdenum films or coatings on various substrates. The powdersmay also be readily used in a wide variety of consolidation processes,such as cold and hot isostatic pressing processes as well as pressingand sintering processes. The high flowability allows the powdersdisclosed herein to readily fill mold cavities, whereas the highdensities minimizes shrinkage that may occur during subsequentsintering. Sintering can be accomplished by heating in an inertatmosphere or in hydrogen to further reduce oxygen content of thecompact.

In another embodiment, the sodium/molybdenum composite metal powders maybe used to form sputter targets, which may then be used in subsequentsputter deposition processes to form sodium/molybdenum films andcoatings. In one embodiment, such sodium/molybdenum films may used toincrease the energy conversion efficiencies of photovoltaic cells.

Having briefly described the sodium/molybdenum composite metal powders12 of the present invention, methods for producing them, and how theymay be used to produce sodium/molybdenum coatings or films onsubstrates, various embodiments of the composite powders, as well asmethods for producing and using the composite powders will now bedescribed in detail.

Referring back now primarily to FIG. 1, a method 10 for producingsodium/molybdenum composite powders 12 may comprise a supply ofmolybdenum metal powder 14 and a supply of a sodium compound 16. Themolybdenum metal power 14 may comprise a molybdenum metal powder havinga particle size in a range of about 0.1 μm to about 15 μm, althoughmolybdenum metal powders 14 having other sizes may also be used.Molybdenum metal powders suitable for use in the present invention arecommercially available from Climax Molybdenum, a Freeport-McMoRanCompany, and from Climax Molybdenum Company, a Freeport-McMoRan Company,Ft. Madison Operations, Ft. Madison, Iowa (US). Alternatively,molybdenum metal powders from other sources may be used as well.

The sodium compound 16 may comprise sodium molybdate, either in itsanhydrous form (i.e., Na₂MoO₄) or as the dihydrate (i.e., Na₂MoO₄.2H₂O),although other sodium-containing materials including, but not limited toelemental sodium, Na₂O, and Na(OH), may be used. Sodium molybdate isusually available in powder form and may comprise any of a wide range ofsizes. The particle size of the sodium molybdate powder 16 is notparticularly critical in embodiments wherein water is used as the liquid18, because sodium molybdate is soluable in water. Sodium molybdatepowders suitable for use in the present invention are commerciallyavailable from Climax Molybdenum, a Freeport-McMoRan Company, Ft.Madison Operations, of Ft. Madison, Iowa (US). Alternatively, sodiummolybdate may be obtained from other sources.

The molybdenum metal powder 14 and sodium molybdate 16 may be mixed witha liquid 18 to form a slurry 20. Generally speaking, the liquid 18 maycomprise deionized water, although other liquids, such as alcohols,volatile liquids, organic liquids, and various mixtures thereof, mayalso be used, as would become apparent to persons having ordinary skillin the art after having become familiar with the teachings providedherein. Consequently, the present invention should not be regarded aslimited to the particular liquids 18 described herein. In addition tothe liquid 18, a binder 48 may be used as well, although the addition ofa binder 48 is not required. Binders 48 suitable for use in the presentinvention include, but are not limited to, polyvinyl alcohol (PVA),Carbowax, and mixtures thereof. The binder 48 may be mixed with theliquid 18 before adding the molybdenum metal powder 14 and the sodiummolybdate 16. Alternatively, the binder 48 could be added to the slurry20, i.e., after the molybdenum metal 14 and sodium molybdate 16 havebeen combined with liquid 18.

The slurry 20 may comprise from about 15% to about 25% by weight liquid(e.g., either liquid 18 alone, or liquid 18 combined with binder 48),with the balance comprising the molybdenum metal powder 14 and thesodium compound 16. The sodium compound 16 (e.g., sodium molybdate) maybe added in amounts suitable to provide the composite metal powder 12and/or final product with the desired amount of “retained” sodium.Because the amount of retained sodium will vary depending on a widerange of factors, the present invention should not be regarded aslimited to the provision of the sodium compound 16 in any particularamounts. Factors that may affect the amount of sodium compound 16 thatis to be provided in slurry 20 include, but are not limited to, theparticular product that is to be produced, the particular “downstream”processes that may be employed, e.g., depending on whether thesodium/molybdenum composite metal powder 12 is sintered, and on whetherthe desired quantity of retained sodium is to be in the powder feedstock(e.g., 24) or in a deposited film or coating (e.g., 32, 32′, 32″).However, by way of example, the mixture of molybdenum metal 14 andsodium molybdate 16 may comprise from about 1% by weight to about 15% byweight sodium molybdate 18. Overall, then, slurry 20 may comprise fromabout 0% by weight (i.e., no binder) to about 2% by weight binder 48.The balance of slurry 20 may comprise molybdenum metal powder 14 (e.g.,in amounts ranging from about 58% by weight to about 84% by weight) andsodium molybdate 16 (e.g., in amounts ranging from about 1% by weight toabout 15% by weight).

Slurry 22 may then be spray dried by any of a wide range of processesthat are now known in the art or that may be developed in the future inorder to produce the composite metal powder product 12, as would becomeapparent to persons having ordinary skill in the art after having becomefamiliar with the teachings provided herein. Consequently, the presentinvention should not be regarded as limited to any particular dryingprocess. However, by way of example, in one embodiment, the slurry 20 isspray dried in a pulse combustion spray dryer 22. More specifically,pulse combustion spray dryer 22 may be of the type shown and describedin U.S. Patent Application Publication No. US 2006/0219056, of Larink,Jr., entitled “Metal Powders and Methods for Producing the Same,” whichis specifically incorporated herein by reference for all that itdiscloses.

Referring now to FIGS. 1 and 6, slurry 20 may be fed into pulsecombustion spray dryer 22, whereupon slurry 20 impinges a stream of hotgas (or gases) 50, which are pulsed at or near sonic speeds. The sonicpulses of hot gas 50 contact the slurry 20 and drive-off substantiallyall of the water and form the composite metal powder product 12. Thetemperature of the pulsating stream of hot gas 50 may be in a range ofabout 300° C. to about 800° C., such as about 465° C. to about 537° C.,and more preferably about 500° C. Generally speaking, the temperature ofthe pulsating stream of hot gas 50 is below the melting point of theslurry constituents, but not below the melting point of elementalsodium. However, the slurry 20 is usually not in contact with the hotgases 50 long enough to transfer a significant amount of heat to theslurry 20, which is significant because of the low melting point ofsodium metal. For example, in a typical embodiment, it is estimated thatthe slurry 20 is generally heated to a temperature in the range of about93° C. to about 121° C. during contact with the pulsating stream of hotgas 50.

As mentioned above, the pulsating stream of hot gas 50 may be producedby a pulse combustion system 22 of the type that is well-known in theart and readily commercially available. By way of example, in oneembodiment, the pulse combustion system 22 may comprise a pulsecombustion system of the type shown and described in U.S. PatentApplication Publication No. 2006/0219056. Referring now to FIG. 6,combustion air 51 may be fed (e.g., pumped) through an inlet 52 into theouter shell 54 of the pulse combustion system 22 at low pressure,whereupon it flows through a unidirectional air valve 56. The air thenenters a tuned combustion chamber 58 where fuel is added via fuel valvesor ports 60. The fuel-air mixture is then ignited by a pilot 62,creating a pulsating stream of hot combustion gases 64 which may bepressurized to a variety of pressures, e.g., in a range of about 15,000Pa (about 2.2 psi) to about 20,000 Pa (about 3 psi) above the combustionfan pressure. The pulsating stream of hot combustion gases 64 rushesdown tailpipe 66 toward the atomizer 68. Just above the atomizer 68,quench air 70 may be fed through an inlet 72 and may be blended with thehot combustion gases 64 in order to attain a pulsating stream of hotgases 50 having the desired temperature. The slurry 20 is introducedinto the pulsating stream of hot gases 50 via the atomizer 68. Theatomized slurry may then disperse in the conical outlet 74 andthereafter enter a conventional tall-form drying chamber (not shown).Further downstream, the composite metal powder product 12 may berecovered using standard collection equipment, such as cyclones and/orbaghouses (also not shown).

In pulsed operation, the air valve 56 is cycled open and closed toalternately let air into the combustion chamber 58 and close for thecombustion thereof. In such cycling, the air valve 56 may be reopenedfor a subsequent pulse just after the previous combustion episode. Thereopening then allows a subsequent air charge (e.g., combustion air 51)to enter. The fuel valve 60 then re-admits fuel, and the mixtureauto-ignites in the combustion chamber 58, as described above. Thiscycle of opening and closing the air valve 56 and combusting the fuel inthe chamber 58 in a pulsing fashion may be controllable at variousfrequencies, e.g., from about 80 Hz to about 110 Hz, although otherfrequencies may also be used.

The “green” sodium/molybdenum composite metal powder product 12 producedby the pulse combustion spray drying process described herein isillustrated in FIGS. 4, 5 a, and 5 b, and comprises a plurality ofgenerally spherically-shaped particles that are themselvesagglomerations of smaller particles. As already described, the sodium ishighly dispersed within the molybdenum, comprising a substantiallyhomogeneous dispersion or composite mixture of sodium and molybdenumsub-particles that are fused together. More specifically, FIG. 5 a is aspectral map produced by energy dispersive x-ray spectroscopy (“EDS”)that illustrates the presence of sodium within the sample of thecomposite metal material 12 that is shown in FIG. 4. FIG. 5 b is aspectral map produced by energy dispersive x-ray spectroscopy that showsthe presence of molybdenum within the sample. As can be seen bycomparing FIGS. 4 and 5 a and 5 b, the sodium is generally evenly andwidely dispersed throughout the composite metal powder product 12.

Generally speaking, the composite metal powder product 12 produced inaccordance with the teachings provided herein will comprise a wide rangeof sizes, and particles having sizes ranging from about 1 μm to about100 μm, such as, for example, sizes ranging from about 5 μm to about 45μm and from about 45 μm to about 90 μm, can be readily produced by thefollowing the teachings provided herein. The composite metal powderproduct 12 may be classified e.g., at step 28 (FIG. 2), if desired, toprovide a product 12 having a more narrow size range. Sieve analyses ofvarious exemplary composite metal powder products 12 are provided inFIG. 7, which is a plot of the particle size distributions (by U.S.Tyler mesh) of the “green” composite metal powder product 12 produced byslurry compositions comprising 3, 7, 9, and 15% by weight sodiummolybdate 18.

As mentioned above, the sodium/molybdenum composite metal powder 12 isalso of high density and is generally quite flowable. Exemplarycomposite metal powder products 12 have Scott densities (i.e., apparentdensities) in a range of about 2 g/cc to about 3 g/cc, as identified inthe various Examples set forth herein. Hall flowabilities range fromabout 35 s/50 g to as low as 30 s/50 g, again as identified in thevarious Examples set forth herein. One example composition (i.e.,Example 12) had no flow, however.

As already described, the pulse combustion system 22 provides apulsating stream of hot gases 50 into which is fed the slurry 20. Thecontact zone and contact time are very short, the time of contact oftenbeing on the order of a fraction of a microsecond. Thus, the physicalinteractions of hot gas 50, sonic waves, and slurry 20 produces thecomposite metal powder product 12. More specifically, the liquidcomponent 18 of slurry 20 is substantially removed or driven away by thesonic (or near sonic) pulse waves of hot gas 50. The short contact timealso ensures that the slurry components are minimally heated, e.g., tolevels on the order of about 93° C. to about 121° C. at the end of thecontact time, temperatures which are sufficient to evaporate the liquidcomponent 18.

In certain instances, residual amounts of liquid (e.g., liquid 18 and/orbinder 48, if used) may remain in the resulting “green” composite metalpowder product 12. Any remaining liquid may be driven-off (e.g.,partially or entirely), by a subsequent sintering or heating step 26.See FIG. 2. Generally speaking, the heating or sintering process 26 isconducted at a moderate temperatures in order to drive off the liquidcomponents and oxygen, but not substantial quantities of sodium. Somesodium may be lost during heating 26, which will reduce the amount ofretained sodium in the sintered or feedstock product 24. It is alsogenerally preferred, but not required, to conduct the heating 26 in ahydrogen atmosphere in order to minimize oxidation of the compositemetal powder 12. Retained oxygen is low, less than about 6%, andgenerally less than about 2%, as indicated in the Examples providedbelow. Heating 26 may be conducted at temperatures within a range ofabout 500° C. to about 825° C. Alternatively, temperatures as high as1050° C. may be used for short periods of time. However, such highertemperatures will usually reduce the amount of retained sodium in thefinal product.

It may also be noted that the agglomerations of the metal powder productpreferably retain their shapes (in many cases, though not necessarily,substantially spherical), even after the heating step 26. Flowabilitydata (Hall data) in heated and/or green forms are also generally verygood (e.g., in a range of about 30-35 s/50 g), as described relative tothe Examples provided herein.

As noted above, in some instances a variety of sizes of agglomeratedproducts may be produced during the drying process, and it may bedesirable to further separate or classify the composite metal powderproduct 12 into a metal powder product having a size range within adesired product size range. For example, most of the composite metalpowder material produced will comprise particle sizes in a wide range(e.g., from about 1 μm to about 150 μm), with a substantial amount ofproduct being in the range of about 5 μm to about 45 μm (i.e., −325 U.S.Tyler mesh) and again in the range of about 45 μm to 90 μm (i.e.,−170+325 U.S. Tyler mesh). See FIG. 7. A process hereof may yield asubstantial percentage of product in this product size range; however,there may be remainder products, particularly the smaller products,outside the desired product size range which may be recycled through thesystem, though liquid (e.g., water) would again have to be added tocreate an appropriate slurry composition. Such recycling is an optionalalternative (or additional) step or steps.

The composite metal powder 12 may be used in its as-recovered or “green”form as a feedstock 24 for a variety of processes and applications,several of which are shown and described herein, and others of whichwill become apparent to persons having ordinary skill in the art afterhaving become familiar with the teachings provided herein.Alternatively, the “green” composite metal powder product 12 may befurther processed, such as, for example, by heating or sintering 26, byclassification 28, and/or combinations thereof, before being used asfeedstock 24.

As mentioned above, the sodium/molybdenum composite metal powder 12 maybe used in various apparatus and processes to deposit sodium/molybdenumfilms on substrates. In one application, such sodium/molybdenum filmscan be used to advantage in the fabrication of photovoltaic cells. Forexample, it is known that the energy conversion efficiency of a CIGSphotovoltaic cell can be increased if sodium is allowed to diffuse intothe molybdenum layer typically used to form an ohmic contact of thephotovoltaic cell. Such efficiency gains are automatically realized inCIGS structures wherein the molybdenum ohmic contact is deposited on asoda-glass substrate. However, they are not realized in structureswherein soda-glass is not used as a substrate.

Referring now to FIG. 3, a photovoltaic cell 36 may comprise a substrate34 on which a sodium/molybdenum film 32, 32′, 32″ may be deposited. Thesubstrate 34 may comprise any of a wide range of substrates such as, forexample, stainless steel, flexible poly films, or other substratematerials now known in the art or that may be developed in the futurethat are, or would be, suitable for such devices. A sodium/molybdenumfilm 32, 32′, 32″ may then be deposited on the substrate 34 by any of awide range of processes now known in the art or that may be developed inthe future, but utilizing in some form the sodium/molybdenum compositemetal powder material 12. For example, and as will be described infurther detail below, the sodium/molybdenum film may be deposited bythermal spray deposition, by printing, by evaporation, or by sputtering.

Once the sodium/molybdenum film (e.g., 32, 32′, 32″) is deposited onsubstrate 34, an absorber layer 76 may be deposited on thesodium/molybdenum film. By way of example, the absorber layer 76 maycomprise one or more selected from the group consisting of copper,indium, and selenium. The absorber layer 76 may be deposited by any of awide range of methods known in the art or that may be developed in thefuture that are, or would be, suitable for the intended application.Consequently, the present invention should not be regarded as limited toany particular deposition process.

Next, a junction partner layer 78 may be deposited on the absorber layer76. Junction partner layer 78 may comprise one or more selected from thegroup consisting of cadmium sulfide and zinc sulfide. Finally, atransparent conductive oxide layer 80 may be deposited on junctionpartner layer 78 to form the photovoltaic cell 36. Junction partnerlayer 78 and transparent conductive oxide layer 80 may be deposited byany of a wide range processes and methods now known in the art or thatmay be developed in the future that are, or would be, suitable fordepositing these materials. Consequently, the present invention shouldnot be regarded as limited to any particular deposition process. Inaddition, because processes for fabricating CIGS photovoltaic cells areknown in the art (with the exception of providing the sodium/molybdenumfilm on the substrate) and could be readily implemented by personshaving ordinary skill in the art after having become familiar with theteachings of the present invention, the particular fabricationtechniques that may be utilized to construct a CIGS photovoltaic cellwill not be described in further detail herein.

As mentioned above, the sodium/molybdenum layer or film 32, 32′, 32″ maybe deposited by any of a wide range of processes. Generally speaking, itis believed that sodium concentrations of about 1% by weight will besufficient to provide the desired efficiency enhancements. Accordingly,the retained sodium present in the feedstock material 24 may be adjustedor varied as necessary in order to provide the desired level of sodiumin the resulting sodium/molybdenum film 32. Generally speaking, retainedsodium levels ranging from about 0.2% by weight to about 3.5% by weightin the feedstock material 24 will be sufficient to provide the desireddegree of sodium enrichment in the sodium/molybdenum film 32. Asindicated in the Examples, such retained sodium levels (e.g., from about0.2 wt. % to about 3.5 wt. %) may be achieved in “green” and sintered(i.e., heated) feedstock material 24 produced by slurries 20 containingfrom about 3 wt. % to about 15 wt. % sodium molybdate.

In one embodiment, a sodium/molybdenum film 32 may be deposited by athermal spray process 30 utilizing the feedstock material 24. Thermalspray process 30 may be accomplished by using any of a wide variety ofthermal spray guns and operated in accordance with any of a wide rangeof parameters in order to deposit on substrate 34 a sodium/molybdenumfilm 32 having the desired thickness and properties. However, becausethermal spray processes are well known in the art and because personshaving ordinary skill in the art would be capable of utilizing suchprocesses after having become familiar with the teachings providedherein, the particular thermal spray process 30 that may be utilizedwill not be described in further detail herein.

In another embodiment, a sodium/molybdenum film 32′ may be deposited onsubstrate 34 by a printing process 38 utilizing the feedstock material24. Feedstock material 24 may be mixed with a suitable vehicle (notshown) to form an “ink” or “paint” that may then be deposited onsubstrate 34 by any of a wide range of printing processes. Here again,because such printing processes are well known in the art and could bereadily implemented by persons having ordinary skill in the art afterhaving become familiar with the teachings provided herein, theparticular printing process 38 that may be utilized will not bedescribed in further detail herein.

In still another embodiment, a sodium/molybdenum film 32″ may bedeposited on substrate 34 by an evaporation process 39 utilizing thefeedstock material 24. By way of example, in one embodiment, evaporationprocess 39 would involve placing the feedstock material 24 in a crucible(not shown) of a suitable evaporation apparatus (also not shown). Thefeedstock material 24 could be placed in the crucible either in the formof a loose powder, pressed pellets, or other consolidated forms, or inany combination thereof. The feedstock material 24 would the be heatedin the crucible until it evaporates, whereupon the evaporated materialwould be deposited on substrate 34, forming the sodium/molybdenum film32″. Evaporation process 39 may utilize any of a wide range ofevaporation apparatus now known in the art or that may be developed inthe future that could be used to evaporate the feedstock material 24 anddeposit film 32″ on substrate 34. Consequently, the present inventionshould not be regarded as limited to use with any particular evaporationapparatus operated in accordance with any particular parameters.Moreover, because such evaporation apparatus are well known in the artand could be readily implemented by persons having ordinary skill in theart after having become familiar with the teachings provided herein, theparticular evaporation apparatus that may be utilized will not bedescribed in further detail herein.

In yet another embodiment, a sodium/molybdenum film 32′″ may bedeposited on substrate 34 by a sputter deposition process. The feedstockmaterial 24 would be processed or formed into a sputter target 44, whichwould then be sputtered in order to form the film 32′″. Any of a widerange of sputter deposition apparatus that are now known in the art orthat may be developed in the future could be used to sputter depositfilm 32′″ on substrate 34. Consequently, the present invention shouldnot be regarded as limited to use with any particular sputter depositionapparatus operated in accordance with any particular parameters.Moreover, because such sputter deposition apparatus are well known inthe art and could be readily implemented by persons having ordinaryskill in the art after having become familiar with the teachingsprovided herein, the particular sputter deposition apparatus that may beutilized will not be described in further detail herein.

As mentioned, the sputter target 44 may comprise a metal product 42 thatmay be fabricated by consolidating or forming the sodium/molybdenumcomposite metal powder 12 at step 40. Alternatively, the sputter target44 could be formed by thermal spraying 30. If the sputter target 44 isto be fabricated by consolidation 40, the feedstock material 24, ineither its “green” form or processed form, may be consolidated or formedin step 40 to produce the metal product (e.g., sputter target 44). Theconsolidation process 40 may comprise any of a wide range of compaction,pressing, and forming processes now known in the art or that may bedeveloped in the future that would be suitable for the particularapplication. Consequently, the present invention should not be regardedas limited to any particular consolidation process.

By way of example, the consolidation process 40 may comprise any of awide range of cold isostatic pressing processes or any of a wide rangeof hot isostatic pressing processes that are well-known in the art. Asis known, both cold and hot isostatic pressing processes generallyinvolve the application of considerable pressure and heat (in the caseof hot isostatic pressing) in order to consolidate or form the compositemetal powder feedstock material 24 into the desired shape. Hot isostaticpressing processes may be conducted at temperatures of 900° C. orgreater, depending on the green density of the sodium/molybdenumcomposite metal powder compact and the retained sodium loss that couldbe tolerated in the final product.

After consolidation 40, the resulting metal product 42 (e.g., sputtertarget 44) may be used “as is” or may be further processed. For example,the metal product 42 may be heated or sintered at step 46 in order tofurther increase the density of the metal product 42. It may bedesirable to conduct such a heating process 46 in a hydrogen atmospherein order to minimize the likelihood that the metal product 42 willbecome oxidized. Generally speaking, it will be preferred to conductsuch heating at temperatures below about 825° C. as higher temperaturesmay result in substantial reductions in the amount of retained sodium,although higher temperatures (e.g., temperatures of 1050° C. or greater)could be used. The resulting metal product 42 may also be machined ifnecessary or desired before being placed in service. Such machiningcould be done regardless of whether the final product 42 was sintered.

EXAMPLES

Several examples have been run using molybdenum metal and sodiummolybdate powders 14, 16, specified herein and available from ClimaxMolybdenum and/or Climax Molybdenum, Ft. Madison Operations. Variousratios of the powders 14 and 16 were combined with deionized water toform the slurries 20. More specifically, the slurries 20 utilized forthe various examples comprised about 20% by weight water (i.e., liquid18), with the remainder being molybdenum metal and sodium molybdatepowders. The ratio of molybdenum metal powder to sodium molybdate wasvaried in the various examples to range from about 3% by weight to about15% by weight sodium molybdate. More specifically, the Examples involvedamounts of 3, 7, 9, and 15 weight percent sodium molybdate.

The slurries 20 were then fed into the pulse combustion spray dryingsystem 22 in the manner described herein. The temperature of thepulsating stream of hot gases 50 was controlled to be within a range ofabout 465° C. to about 537° C. The pulsating stream of hot gases 50produced by the pulse combustion system 22 substantially drove-off thewater from the slurry 20 to form the composite metal powder product 12.The contact zone and contact time were very short, the contact zone onthe order of about 5.1 cm and the time of contact being on the order of0.2 microseconds.

The resulting metal powder product 12 comprised agglomerations ofsmaller particles that were substantially solid (i.e., not hollow) andhaving generally spherical shapes. An SEM photo of a “green”sodium/molybdenum composite metal powder produced by a slurry 20comprising 9% by weight sodium molybdate is presented in FIG. 4. Data inTables I and II are presented for the various examples in both “green”form, and after being sintered or heated in a hydrogen atmosphere at thetemperatures and for the times specified. Data are also presented forscreened green material (+325 mesh moly) as also indicated in Tables Iand II.

TABLE I Molyb- Sodium Hall denum Molybdate Apparent flow Exam- metal(SoMo) Density (s/ ple Batch (wt %) (wt %) (g/cc) 50 g) 1 3% SoMo Green97% 3% 2 7% SoMo Green 93% 7% 2.89 33 3 9% SoMo Green 91% 9% 4 15% SoMoGreen 85% 15% 5 3% SoMo + 325 mesh 97% 3% 6 9% SoMo + 325 mesh 91% 9% 715% SoMo + 325 85% 15% mesh 8 9% SoMo Sintered 91% 9% 1 h. 1050° C. 9 7%SoMo Sintered 93% 7% 2.79 32 10 h. 640° C. 10 9% SoMo Sintered 91% 9% 10h. 640° C. 11 3% SoMo Sintered 97% 3% 6 h. 825° C. 12 9% SoMo Sintered91% 9% 2.62 No 6 h. 825° C. Flow 13 15% SoMo Sintered 85% 15% 6 h. 825°C.

TABLE II Slurry % Weight Viscosity Sodium loss Na Sec. Conc. duringDistribution Example Zahn#1 (wt. %) % O % N Sintering (EDS) 1 33.8 0.74%1.23% 0.0020% 2 1.39% 2.14% 0.2500% 3 35 1.74% 2.64% 0.0075% Na variesin particles from 3% to 12% 4 3.11% 5.58% 0.0295% 5 1.22% 0.0016% 61.84% 2.93% 0.0101% No Na peak found by EDS 7 3.09% 5.16% 0.196% 8 0.73%No Na peak found by EDS 9 1.36% 1.36% 0.0000% 0.97% 10 1.85% 1.85%0.0018% 1.85%   4% 11 0.22% 0.22% 0.0011% 1.79% 12 1.32% 1.32% 0.0010%3.90% 1.86% 13 2.39% 2.39% 0.0015% 4.84% 2.89%

Having herein set forth preferred embodiments of the present invention,it is anticipated that suitable modifications can be made thereto whichwill nonetheless remain within the scope of the invention. The inventionshall therefore only be construed in accordance with the followingclaims:

1. A method for producing a composite metal powder, comprising:providing a supply of molybdenum metal powder; providing a supply of asodium compound; combining said molybdenum metal powder and said sodiumcompound with a liquid to form a slurry; feeding said slurry into astream of hot gas; and recovering the composite metal powder, saidcomposite metal powder comprising sodium and molybdenum.
 2. The methodof claim 1, wherein providing a supply of a sodium compound comprisesproviding a supply of sodium molybdate powder.
 3. The method of claim 1,wherein feeding said slurry into a stream of hot gas comprises atomizingsaid slurry and contacting said atomized slurry with the stream of hotgas.
 4. The method of claim 1, wherein combining said molybdenum metalpowder and said sodium compound with a liquid comprises combining saidmolybdenum metal powder and said sodium compound with water to form aslurry.
 5. The method of claim 1, wherein said slurry comprises betweenabout 15% to about 25% by weight liquid.
 6. The method of claim 1,further comprising: providing a supply of a binder material; andcombining said binder material with said molybdenum metal powder, saidsodium compound, and said water to form a slurry.
 7. The method of claim6, wherein said binder comprises one or more from the selected from thegroup consisting of polyvinyl alcohol and carbowax.
 8. The method ofclaim 6, wherein said sodium compound comprises sodium molybdate andwherein said slurry comprises between about 15% to about 25% by weightliquid, from about 0% to about 2% by weight binder, from about 1% byweight to about 15% sodium molybdate, and from about 58% by weight toabout 84% by weight molybdenum metal powder.
 9. The method of claim 6,further comprising heating the recovered composite metal powder at atemperature sufficient to drive-off substantially all of said binder.10. The method of claim 9, wherein said heating further comprisesheating in a hydrogen atmosphere.
 11. The method of claim 10, whereinsaid heating in a hydrogen atmosphere is conducted at a temperature in arange of about 500° C. to about 825° C.
 12. A sodium/molybdenumcomposite metal powder comprising a substantially homogeneous dispersionof sodium and molybdenum sub-particles that are fused together to formindividual particles of said composite metal powder.
 13. Thesodium/molybdenum composite metal powder of claim comprising a Hallflowability in a range of about 30-35 seconds for 50 grams.
 14. Thesodium/molybdenum composite metal powder product of claim 12 having aScott density in a range of about 2 g/cc to about 3 g/cc.
 15. Thesodium/molybdenum composite metal powder product of claim 12, comprisingfrom about 0.2% by weight to about 3.5% by weight retained sodium. 16.The sodium/molybdenum composite metal powder product of claim 12comprising less than about 6% by weight retained oxygen.
 17. A methodfor producing a metal article, comprising: producing a supply of acomposite metal powder by: providing a supply of molybdenum metalpowder; providing a supply of a sodium compound; combining saidmolybdenum metal powder and said sodium compound with a liquid to form aslurry; feeding said slurry into a stream of hot gas; recovering thecomposite metal powder; and consolidating said composite metal powder toform the metal article, said metal article comprising asodium/molybdenum metal matrix.
 18. The method of claim 17, wherein saidconsolidating said composite metal powder comprises cold isostaticpressing.
 19. The method of claim 17, wherein consolidating comprisespressing said composite metal powder into a shape and sintering theshape.
 20. The method of claim 19, wherein said sintering is conductedin a hydrogen atmosphere.
 21. The method of claim 20, wherein saidsintering is conducted at a temperature of less than about 825° C. 22.The method of claim 17, wherein consolidating comprises hot isostaticpressing.
 23. A method for producing a photovoltaic cell, comprising:providing a substrate; depositing a sodium/molybdenum metal layer onsaid substrate; depositing an absorber layer on said sodium/molybdenummetal layer; and depositing a junction partner layer on said absorberlayer; wherein depositing a sodium/molybdenum metal layer comprises:providing a supply of a composite metal powder comprising molybdenum andsodium; and depositing said composite metal powder on said substrate bythermal spraying, by printing, or by evaporation.
 24. A method fordepositing a film on a substrate, comprising: providing a supply of acomposite metal powder comprising molybdenum and sodium; depositing saidcomposite metal powder on said substrate by thermal spraying.
 25. Amethod for depositing a film on a substrate, comprising sputtering atarget comprising a sodium/molybdenum metal matrix, sputtered materialfrom the target forming said sodium/molybdenum metal layer.
 26. A methodfor forming a coating on a substrate, comprising: providing a supply ofa composite metal powder comprising molybdenum and sodium; mixing saidsupply of composite metal powder with a vehicle; and depositing themixture of the composite metal powder and the vehicle on said substrateby printing.
 27. A method for depositing a film on a substrate,comprising: providing a supply of a composite metal powder comprisingmolybdenum and sodium; depositing said composite metal powder on saidsubstrate by evaporation.